1. Field of the Invention
The present invention relates generally to a storage apparatus, seek control method and program that move a head to an arbitrary target track on a medium to perform writing or reading, and, more particularly, to a storage apparatus, seek control method and program that enhance a seek performance by reducing a settling time with a position control when the head arrives at the target track.
2. Description of the Related Art
Conventionally, in seek control of a magnetic disk apparatus, a target speed profile is generated based on the number of remaining tracks (track difference) relative to a target track to control a speed of a head actuator by driving with a voice coil motor; when the number of remaining tracks relative to the target track is reduced to a predetermined value, a position control is started to position a head to a track center; when arriving at the target track, the speed control is discontinued to enable only the position control; and if the head converges within a settling slice (settling threshold) defining a predetermined width centering on the track center within a predetermined settling time, an access permission signal is enabled to start a write operation or read operation for a disk medium. If the head does not converge within the settling slice within the settling time, since a seek error occurs when the settling time has elapsed and the seek control is retried, the seek time is prolonged and the performance is deteriorated. In such a conventional seek control, to reduce the remaining oscillations of the head when the head arrives at the target track and the position control is enabled, the settling time is extended in consideration of fluctuations of bias currents keeping the head actuator at the target track, which are caused by mechanical variations of a head driving mechanism and a seek distance.
FIGS. 1A and 1B show a behavior of a head position when switched to a conventional position control. FIG. 1A is a head position signal; the seek control is started at time t1; when arriving at the target track at time t2, the position control is enabled; the head position signal converges within a settling slice 200-1, 200-2 as shown by an oscillating waveform 202 at time t3; at this point of time, an access permission signal of FIG. 1B becomes H-level; and the reading or the writing can be performed for a disk medium. In this case, the seek time is T1. Due to the variations of the head driving mechanism and the seek distance, the head converges within the settling slice 200-1, 200-2 at time t4 with a substantial change as shown by an oscillating waveform 204, and the seek time is extended to T2 to generate a variation of ΔT. Therefore, conventionally, the seek times until the head converges within the settling slice are measured under various environmental conditions such as temperatures and voltages to set the settling time in anticipation of 6σ(=±3σ) of variation σ in a normal distribution of a multiplicity of the detected seek times, and since the settling slice itself is not narrow and has some margin, a performance of an apparatus has not been affected directly. However, recently, track pitches are narrowed considerably to achieve higher density in tracks-per-inch (TPI) of a disk medium. Therefore, the settling slice is also narrowed considerably along with the higher track density. Consequently, the settling time is further extended to absorb variations of times until the convergence within the settling slice. However, since the seek time is prolonged and the performance is deteriorated if the settling time is extended, to converge the head position oscillations within a certain settling slice more quickly when switched to the position control without extending the settling time, an integral gain is increased in a proportional integral calculation used for the position calculation to allow the head to converge quickly within the settling slice.
FIGS. 2A and 2B show a behavior of a head position signal and an access permission signal when the multiplication constant of the integral gain of the position control is one; the seek control is started at time t1; when arriving at the target track at time t2, the position control is enabled to allow attenuation as shown by a waveform 206; the head position signal converges within the settling slice 200-1, 200-2 at time 4; and the access permission signal becomes H-level.
FIGS. 3A and 3B show a behavior of a head position signal and an access permission signal when the integral gain of the position control is increased n-times; the seek control is started at time t1; when arriving at the target track at time t2, the position control is enabled; rapid attenuation is achieved by increasing the integral gain as shown by a waveform 208; the head position signal converges within the settling slice 200-1, 200-2 at time 3; and the access permission signal becomes H-level. By increasing the integral gain in this way, the head can be quickly pulled in to the track center to reduce the variation anticipated conventionally and, as compared to the case that the integral gain is not increased as shown in FIGS. 2A and 2B, since the seek time can be T2 that is shorter than T1 by ΔT and the settling time is not needed to be extended, the performance is not deteriorated. Such a position control at the time of the settling includes patent documents 1 and 2, for example. The patent document 1 is a servo gain control method at the timer of the settling, which reduces the time for eliminating oscillations by repeating gain-down while determining the oscillations after gain-up. The patent document 2 is the settling determination, which calculates an oscillation component and an optimum settling time from a head movement speed and servo compensating means to achieve time reduction. However, in such a conventional method of reducing the seek time by increasing the integral gain, if the integral gain is increased as shown in FIGS. 5A to 5C, as compared to the waveform 210 when the integral gain is not increased as shown in FIGS. 4A to 4C, the fluctuations of the waveform 212 becomes greater after converging within the settling slice. In other words, if the integral gain is increased, the on-track accuracy to the target track is deteriorated after the seeking is completed.
In FIGS. 16 and 17A to 17C are the head position signals, access permission signals, and integral gains, respectively; the head position signals converge within the settling slice 200-1, 200-2 at time t1; and the access permission signals are raised to H-level. To eliminate the deterioration of the on-track accuracy due to the increase of the integral gain, as shown in FIGS. 5A to 5C, the integral gain must be restored to the original gain at the timing of time t2 during the track following control. However, if the integral gain is restored at time t2, the head position signal fluctuates wildly as shown by oscillations 214 and exceeds the settling slice 200-1, 200-2 to generate an off-track error, which causes a write error or read error during the writing or reading, and it is problematic that the access performance is reduced.